Cooperating Receiver Selection for UMTS Wireless Location

ABSTRACT

For Wireless Communications Networks (WCNs) that support soft handover, cooperator receiver selection for a TDOA, AOA, TDOA/AOA, or hybrid network-based or network-overlay Wireless Location System (WLS) must contend with one or more network base stations as a serving cell. When the active set contains more than one member, two techniques for determining a set of cooperating and demodulating receivers to use in the signal collection for location estimation is disclosed. In one embodiment, the active set members are constructively reduced to a single member that is used as a proxy serving cell. In another embodiment, the information contained in the active set membership is retained and a new set of demodulating and cooperator receivers are generated based on the entire membership of the active set.

CROSS REFERENCE

This application is related to co-pending application entitled“Cooperating Receiver Selection for UMTS Wireless Location,” filed oneven date herewith under Attorney Docket No. TPI-1120.

TECHNICAL FIELD

The subject matter described herein relates generally to methods andsystems for locating wireless devices using cellular radio networks andother types of voice or data wireless communications systems. Moreparticularly, but not exclusively, the subject matter described hereinrelates to the use of measurements provided to the network by the mobileas part of normal operation to facilitate the selection of receivers toparticipate in the use of network-based location techniques to locate amobile communications device in a Code Division Multiple Access spreadspectrum based wireless communications system.

BACKGROUND OF THE INVENTION

Early work relating to network-based wireless location systems isdescribed in commonly assigned U.S. Pat. No. 5,327,144 “CellularTelephone Location System,” which discloses a system for locatingcellular telephones using time difference of arrival (TDOA) techniques.Further enhancements of the system disclosed in the '144 patent aredisclosed in commonly assigned U.S. Pat. No. 5,608,410 “System forLocating a Source of Bursty Transmissions.” Early art in the location ofmobile devices via network-based Angle of Arrival (AoA) and Hybrids ofAoA and TDOA include the commonly held U.S. Pat. Nos. 4,728,959;“Direction finding localization system”; 6,119,013 and 6,108,555 bothentitled “Enhanced time difference localization system.”

Enhancements for network-based wireless location systems for CDMAsystems can be found in the commonly held U.S. Pat. Nos. 7,340,259“Robust, efficient, localization system”; 6,546,256 “Robust, efficient,location-related measurement”; and 6,047,192 “Robust, efficient,localization system.”

The ability of a CDMA-based system to maintain multiple legs between themobile device and the network base station(s) is called “soft hand-off”(IS-95/IS-2000) or “soft-handover” (UMTS).

A soft handoff in a CDMA-based system occurs based on the beacon orpilot signal strength of several sets of base stations as measured bythe mobile device (User Equipment (UMTS) or Mobile Station(IS-95/IS-2000)).

These sets are known in IS-95/IS-2000 variously as the active set, theneighbor set, the candidate set and the remaining set. In a UMTS system,the roughly corresponding sets of NodeB's are deemed the active,monitored and detected sets.

The active set is the set of base stations or NodeB's through whichactive communication is established. This definition of the active setapplies to all aforementioned CDMA-based Wireless Communications System(WCNs).

In IS-95/IS_(—)2000, the neighbor set is a set of base stations inproximity to the active base stations and includes base stations thathave a high probability of having a pilot signal strength of sufficientlevel to establish communication, but through which active communicationis not yet established. The remaining set is a set of base stations thathave mobile detectable pilots, but are not of sufficient quality orpower to be included in any of the other three sets.

In UMTS, in addition to the active set, two other mutually exclusivesets are defined. The “monitored set” includes non-active set cellsnevertheless known to the network. In UMTS, these cells are included bythe UTRAN in broadcast “CELL_INFO_LIST”. “Detected set cells” are thosecells detected by the mobile station (also called the User Equipment orUE), which are not known to the network. In UMTS, these cells are notfound in the CELL_INFO_LIST or in the active set.

In CDMA (IS-95 and IS-2000), the active set members typically havehigher measured pilot signals strengths relative to the neighbor,candidate set and remaining sets. The mobile uses these sets to helpmanage the handover/handoff process known as Mobile-Assisted Handover(MAHO). When communications between the network and mobile are initiallyestablished, a mobile communicates through radio signaling with a singlebase station, typically the base station with the highest received pilotpower, but always a base station that meets the threshold for inclusioninto the active set. During soft-handoff, the active set contains morethan one base station. The mobile monitors the pilot signal strength ofthe base stations in the active set, the candidate set, the neighbor setand the remaining set. During handoff, when a pilot signal strength of abase station in the neighbor or remaining set reaches a definedthreshold level, that base station is added to the candidate set andremoved from the neighbor or remaining set by the mobile. When themobile detects a relatively strong candidate pilot, the UE transmits a“Pilot Strength Measurement Message” (PSMM) to a Base StationController/Packet Control Unit (BSC/PCU) along with a request to add thebase station of that pilot signal to the UE's active set. The PSMMreport is evaluated by the BSC which coordinates the processing of asoft handoff with the base stations associated with the strong detectedpilot signals.

In CDMA-based systems wireless communications systems, using the UMTSWCN as an example and source of nomenclature, the concept of a ‘servingcell’ has been replaced with one-way, two-way, three-way, etc.softhandoff handover (SHO) to take advantage of macrodiversity. In thedownlink (NodeB-to-UE), macrodiversity is accomplished by combining, inthe mobile's RAKE receiver, multiple copies of the downlink signalcaused by either transmission from multiple antennas or by themulti-path corruption of the transmitted signal.

In the uplink direction, macrodiversity is accomplished through the useof multiple receive antennas collecting multiple copies of the UEtransmitted signal. Since the UE transmitted signal is multi-pathcorrupted, multiple levels of signal combining can take place.

In all CDMA-based radio air interface wireless communications systems,detection of surrounding cell beacons is complicated by frequency re-useand the power control used to minimize the Near-Far effects.

The near-far problem is a classic co-channel interference (also calledcross-talk) problem in cellular frequency reuse radio networks. Thenear-far problem arises from the fact that radio signals fromtransmitters closer to the receiver of interest are received withsmaller radio path-loss attenuation than are signals from transmitterslocated further away. Therefore the strong signal from the nearbytransmitter will mask the weak signal from the more distant transmitter.

In CDMA-based radio networks, the near-far co-channel interference isactively minimized using dynamic output power adjustment of thetransmitters both in the uplink (UE-to-NodeB) and downlink (NodeB-to-UE)directions. With dynamic output power adjustment the closer transmitters(with less radio path loss) broadcast with less power so that the SNRfor all transmitters at the serving receiver is roughly the same.

A network Wireless Location Services scenario may include hybrids withdownlink and Satellite location techniques for a CDMA-based wirelesscommunications network (WCN) such as the Universal Mobile TelephoneSystem (UMTS). The UMTS WCN is fully specified by the 3rd GenerationPartnership Project (3GPP) since December 1998.

Detailed descriptions of Radio messages, message elements, andparameters for UMTS can be found in technical specification document3GPP TS 24.008 “Mobile radio interface Layer 3 specification; Corenetwork protocols; Stage 3” and 3GPP TS 25.331 “Radio Resource Control(RRC); Protocol specification”

Detailed descriptions of the Wireless Location Systems standardized forUMTS are detailed in technical specification 3GPP TS 25.305 “UserEquipment (UE) positioning in Universal Terrestrial Radio Access Network(UTRAN); Stage 2”. Details on handovers in the exemplary UMTS networkcan be found in 3GPP TS 23.009; “Handover Procedures”, 3GPP TR 25.832;“Manifestations of Handover and SRNS Relocation” and 3GPP TR 25.936;“Handovers for real time services from PS domain”.

The ETSI and 3GPP defined term LMU (Location Mobile Unit) isfunctionally equivalent to the ANSI defined term Position DeterminingUnit (PDE) or to the Signal Collection System (SCS) term as used in thecited TruePosition Patents. In a network-based WLS, consisting ofgeographically distributed receivers (LMUs) either overlaid in orintegral to the local Wireless Communications Network with centralserver(s), the Serving-Mobile Location Center (SMC) connects to the corecommunications network. The central server(s) communicate with the WCNfor the purposes of obtaining location triggers and collecting locationtasking information which in this case includes the Active Set of themobile of interest.

Prior U-TDOA systems required that at least one receiver, deemed thereference LMU receiver, to successfully demodulate at least part of thesignal from the mobile of interest.

In a CDMA-based WCN with soft-handover, more than one LMU may be able tofully or partially demodulate the signal from the mobile of interest.The resultant full or partial signal demodulations may, viasoft-combining, be used to reconstruct a replica of the originaltransmission that is less degraded than the best replica that could beobtained from any of the individual demodulations. This reconstructedreference signal is then made available to all LMUs involved in thelocation for correlation processing. LMUs that participate in thedemodulation process are called “demodulating LMUs” or “demod LMUs.” Inaddition to the demodulating LMU receivers, geographically neighboringor proximate LMUs (“cooperators” or “coop LMUs”) may be tasked tocollect signals from the mobile of interest for correlation with thereference signal. These cooperators may be an LMU, an LMU sector, ormultiple antennas serving the same LMU. The set of potential cooperatingLMUs also includes the demodulating LMUs. A “demod sector” is one LMUsector that is tasked for demodulation. A “coop sector” is one LMUsector that is tasked for cooperation. The problem of identifying whichLMU sectors to task for demodulation is related to, but separate from,the problem of identifying which LMU sectors to task for cooperation.Although the techniques described herein may solve both problems, theymay also be used to solving either problem independently of the other.Thus, for example, in an embodiment in which the WLS is not required tocollect a reference signal because the reference signal is provided tothe WLS by the WCN, the techniques described herein can be used toidentify which LMUs to task for cooperation. In such a case the LMUsidentified as demod LMUs by these techniques would be used only forcooperation and not for demodulation. Not every sector or cell may havea LMU installed (e.g., a sparse network deployment, as described inTruePosition's U.S. patent application Ser. Nos. 11/736,950, 11/736,920,11/736,868 and 11/736,902; all entitled “Sparsed U-TDOA WirelessLocation Networks”). Both coop sectors and demod sectors are limited tothose sectors and cells that have an associated LMU.

Thus “serving sector” may refer to the coverage area of the servingcell. The term “LMU sector” may be used for that portion of an LMUresponsible for receiving and processing radio signals from one receiveantenna if receive diversity is not in use, or from multiple antennaslocated in close proximity to one another and providing diversitycoverage of the same area if receive diversity is in use.

For network-based wireless location systems operating in a CDMA-basedwireless communications network, selection of a most nearly optimalgroup of uplink receivers for network-based wireless location isproblematic due to the power-control inherent in such networks and theresulting near far problem.

SUMMARY

For the wireless location system to dynamically task the numericallysmallest, most geographically favorable set of receivers for a reliableTDOA and/or AoA based location and velocity calculation, the mobiledevice's Active Set may be obtained from the Wireless CommunicationsNetwork and used to select from pre-determined lists of cooperatingreceivers and demodulating receivers or to construct new lists ofcooperating receivers and demodulating receivers.

For Wireless Communications Networks (WCNs) that support soft handover,cooperator receiver selection for a TDOA, AOA, TDOA/AOA, or hybridnetwork-based or network-overlay Wireless Location System (WLS) mustcontend with one or more network base stations as a serving cell. In aWCN that supports soft-handoff, such as a CDMA (Code Division MultipleAccess) based system, which can be a FDD (Frequency Division Duplex) ora TDD (Time Division Duplex) system, the concept of a serving cell orserving sector is more complicated. First, the mobile device may havemultiple serving sectors (also known as active set members). Second,each sector may have differing numbers of transmission and receptionantennae or, in TDD-based systems, use the same antenna for transmissionand reception. When the active set contains a single member, thesolution is straightforward and a cooperative receiver may be selectedusing a number of criteria or methods. When the active set contains morethan one member, disclosed herein are two techniques for determining aset of cooperators and demodulators to use in the signal collection forlocation estimation.

-   -   1. In one embodiment, a first technique is referred to as the        proxy method, since the active set members are constructively        reduced to a single member that is used as a proxy serving cell.    -   2. In another embodiment, a second technique is referred to as        the aggregate method, since information contained in the active        set membership is retained and a new set of demodulating and        cooperator receivers are generated based on the entire        membership of the active set.

In both techniques, the selected cooperative and demodulating receiversare those that are likely to provide good TDOA and AoA coverage for themobile device.

CDMA mobile stations and UMTS User Equipment are based on wide-band airinterfaces and transmit at very low Eb/N0 levels compared to othernarrow-band air interfaces (including GSM, TDMA and AMPS) for whichUTDOA location systems have been widely deployed. To date, UTDOAdeployments in CDMA/UMTS have been small enough to be practical to useevery LMU receiver as a cooperator and a demodulator to each other LMU.

Due to the low Eb/N0 levels that are used, less margin of error isavailable for demodulating receiver and cooperating receiver selectionwhen locating UMTS User Equipment. The disclosed techniques forselecting demodulating and cooperating receivers provide a mechanism forselection of demodulating and cooperating receivers that allow aCDMA-based UTDOA system to achieve the same or better location accuracycompared to accuracies achieved by comparable UTDOA systems for otherair interfaces. The disclosed techniques may use fewer cooperators, thusallowing higher system throughput. Because the number of possibledistinct Active Sets is extraordinarily high, it may not be possible toperform this selection in advance for all cases that may be encountered.Hence, any practical solution may benefit from the disclosed techniques.

The disclosed techniques are also applicable to a hybrid solution thatuse mobile-based OTDOA and/or assisted GPS (A-GPS) and network basedUplink Time Difference of Arrival (U-TDOA) technologies. Suchtechnologies operate independently to obtain range estimates which canthen be combined in a final hybrid location calculation or operate in afallback mode where one location method is used when one or more of theother methods fail. Use of a hybrid wireless location system, using thedisclosed concepts, creates an improved location solution with enhancedaccuracy, yield, and performance. Methods of using network-based withmobile-based technologies, including satellite based downlink TDOA, weredisclosed in TruePosition U.S. Pat. No. 7,440,762 “TDOA/GPS hybridwireless location system” and in TruePosition U.S. patent applicationSer. No. 12/192,057 “Hybrid GNSS and TDOA Wireless Location System.”

The inventive techniques and concepts described herein apply tocode-division radio communications systems such as the CDMAOne (TIA/EIAIS-95 CDMA with IS-95A and IS-95B revisions), the CDMA2000 family ofradio protocols (as defined by the 3rd Generation Partnership Project 2(3GPP2)) and the Wideband Code-Division Multiple-Access (W-CDMA) radiosystem defined by the 3rd Generation Partnership Project (3GPP) as partof the Universal Mobile Telephone System (UMTS). The UMTS modeldiscussed herein is an exemplary but not exclusive environment in whichthe present invention may be used.

The present invention may be used in a network Wireless LocationServices scenario including hybrids with downlink and satellite locationtechniques for a CDMA-based wireless communications network (WCN) suchas the Universal Mobile Telephone System (UMTS). The UMTS WCN has beenfully specified by the 3rd Generation Partnership Project (3GPP) sinceDecember 1998. The UMTS WCN with its Wideband CDMA (W-CDMA) radio airinterface, also specified by 3GPP, will be used as an exemplary modelthroughout this document.

It should be noted that this summary is provided to introduce aselection of concepts in a simplified form that are further describedbelow. This summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used as an aid in determining the scope of the claimed subjectmatter.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary as well as the following detailed description isbetter understood when read in conjunction with the appended drawings.For the purpose of illustrating the invention, there is shown in thedrawings exemplary constructions of the invention; however, theinvention is not limited to the specific methods and instrumentalitiesdisclosed. In the drawings:

FIG. 1 illustrates a network-based wireless location system (WLS).

FIG. 2 a depicts an example selection of representative marker points inan omni-directional cell.

FIG. 2 b depicts an example selection of representative marker points ina cell in a sectored cell site.

FIG. 2 c depicts the radial segmentation and spiral algorithm patternfor cooperative and demodulating LMU sector selection.

FIG. 2 d depicts the radio propagation study output of the averagesignal quality between each serving cell and its target cooperating anddemodulating LMU sectors in a service area.

FIG. 2 e illustrates the spiral algorithm in process, selecting the bestcooperating and demodulating LMU sectors in order of radial segmentationand predicted signal quality.

FIG. 2 f illustrates a representation of the initial cooperator lists.

FIG. 2 g illustrates a representation of the initial demodulating LMUsector lists.

FIG. 3 illustrates the general operative steps in performing a ProxyMethod for selection of cooperative and demodulating LMU sectors for anetwork-based wireless location system.

FIG. 4 illustrates an embodiment of Proxy Method 1—Select the Active SetMember Nearest to Centroid as the Proxy

FIG. 5 illustrates an embodiment of Proxy Method 2—Select as Proxy Cellthe Active Set Member with the Most Active Set Members on itsDemodulating LMU List.

FIG. 6 illustrates an embodiment of Proxy Method 3—Select Proxy ServingCell Based on Coverage Bounding Polygons

FIG. 7 details the general operative steps in performing an AggregateMethod for selection of cooperative and demodulating LMU sectors for anetwork-based wireless location system.

FIG. 8 a illustrates an embodiment of Aggregate Method 1—Construction ofnew cooperators in a round-robin fashion based on octants of active setmembers.

FIG. 8 b depicts the Spiral Algorithm as implemented for AggregateMethod 1—Construction of new cooperators in a round-robin fashion basedon octants of active set members.

FIG. 8 c depicts the output of the Spiral Algorithm as implemented forAggregate Method 1—Construction of new cooperators in a round-robinfashion based on octants of active set members.

FIG. 9 a illustrates an embodiment of Aggregate Method 2—Construct newcooperators list based on marker points of active set members

FIG. 9 b depicts the Spiral Algorithm as implemented for AggregateMethod 2—Construct new cooperators list based on marker points of activeset members

FIG. 10 a illustrates an embodiment of Aggregate Method 3, Construct newcoops based on the coverage area common to all active set members.

FIG. 10 b depicts the Spiral Algorithm as implemented for AggregateMethod 3, Construct new coops based on the coverage area common to allactive set members.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Certain specific details are set forth in the following description andfigures to provide a thorough understanding of various embodiments ofthe invention. Certain well-known details often associated withcomputing and software technology are not set forth in the followingdisclosure to avoid unnecessarily obscuring the various embodiments ofthe invention. Further, those of ordinary skill in the relevant art willunderstand that they can practice other embodiments of the inventionwithout one or more of the details described below. Finally, whilevarious methods are described with reference to steps and sequences inthe following disclosure, the description as such is for providing aclear implementation of embodiments of the invention, and the steps andsequences of steps should not be taken as required to practice thisinvention.

We will now describe illustrative embodiments of the present invention.First, we provide a detailed overview of the problem and then a moredetailed description of our solutions.

One of the main advantages of wide-band, spread spectrum CDMA-basedwireless communication systems is the capability of combining multiplesignals that arrive in the receivers with different time delays. Unlikenarrowband systems, CDMA-based wireless systems do not use equalizationto mitigate the negative effects of multipath, but rather combines themultipath signals (known as ‘rays’) using a RAKE receiver.

A RAKE receiver contains multiple demodulators, called ‘fingers’. Eachfinger searches for rays and feeds the information to the other fingersof the RAKE receiver. Each finger then demodulates the signalcorresponding to a strong ray. The results from each finger are thencombined together to improve the signal quality.

In CDMA-based wireless communications systems, the techniques ofsoft-handover (SHO) and softer-handover (also SHO) are used to takeadvantage of the performance enabled by multipath combining allowingimprovement the voice or data quality as delivered with the caprices ofthe radio air interface. Using macrodiversity, signals from the antennasof the local base station can be combined to enhanced the receivedsignal (softer-handover) and signals from other local base stations canbe collected and merged (soft-handover) at the base station or radionetwork controller.

Using the 3GPP defined UMTS system as an example of a functionalwideband, spread spectrum CDMA-based wireless communication network, itcan be shown that network and signal information can be obtained toimprove the performance of the Wireless Location System as to accuracy,latency, and cost.

Network-based Wireless Location Systems require a minimum of threereceivers (or two in the case of Angle-of-Arrival only based systems) tocalculate a location estimate via algorithms known as Triangulation(AoA) and Trilateration (TDOA). In practice, many more receivers aretypically needed to maximize location accuracy.

Network-based WLSs using multi-lateration (and multi-angulation incombination) can improve the location accuracy by selecting only thereceiver sites with the best reception (as determined by the signalquality, e.g., the SNR). Each pair of receiver sites forms a baselinepair from which the location estimation and velocity estimationcalculations are performed. Inconsistencies between the received timingand the expected timing can also be used with SNR as additional orsubstitute receiver selection criteria.

The baseline method for TDOA/FDOA determination in a network-basedwireless location system uses signal correlation between a reference andlocal signal where the reference and local signals are collected overthe same interval, as extended and adjusted for expected propagationdelay, using different antennas via synchronized receivers.

FIG. 1

FIG. 1 depicts a network-based wireless location system (WLS). The WLSoverlays or is integrated into the wireless communications system (shownhere as the strongest cell 102, neighbor cell 103 and proximate cell104). A wireless device 105 is shown in soft-handoff with the localcells 102 103 104 via radio signaling paths 106 107 108. LocationMeasurement Units (LMUs) 109 are geographically distributed to receivethe uplink radio signals emitted by the mobile device 105. The LMUs areconnected via digital data links 110 to the SMLC 111. The SMLC 111 orServing Mobile Location Center manages the operations, maintenance, andprovisioning of the LMUs 109 as well as providing interconnectionbetween the WLS and other wireless communications network nodes. TheSMLC receives triggering and tasking information from the WCN via adigital datalink 112 such as the J-STD-036 defined E5 interface, theETSI defined Lb interface, the 3GPP defined Iupc interface and/or theATIS defined Lbis interface. If deployed in conjunction with a linkmonitoring system (LMS), the SMLC may support an alternate triggeringinterface 113 for triggering and tasking formation from the LMS. The LMSis further detailed in commonly assigned U.S. Pat. Nos. 6,782,264 and7,023,383 both entitled “Monitoring of call information in a wirelesslocation system” and commonly assigned patent application Ser. No.11/150,414 “Advanced triggers for location-based service applications ina wireless location system”.

In the network-based WLS, received signals are collected using widebandreceivers (LMUs 109) preferably using geographically distributed antennasites.

The reference antenna (or site) and cooperators are selected based onreceived signal characteristics, a pre-planned scheme based on signalpropagation modeling or other criteria which can include the servingcell or active set membership for a mobile device.

Each of the LMU 109 receivers digitizes the radio transmission receivedon the channel of interest. The acquired signal of interest (SOI) orportions of the SOI is demodulated by the reference receiver anddistributed to the cooperating sites. Additional information on thespecifics of this technique may be found in the commonly held U.S. Pat.Nos. 5,327,144; “Cellular telephone location system” and 6,047,192;“Robust, efficient, localization system.”

The reference and local signals, a set of digitized samples collectedover the sample duration, are then correlated with a set of likelytime-offsets (range) and frequency-offsets (Doppler and drift) to createa three dimensional search space of correlation amplitude, range, andDoppler/drift (as per commonly assigned U.S. Pat. No. 6,876,859; “Methodfor estimating TDOA and FDOA in a wireless location system”).

The said correlation procedure is repeated for each cooperating LMU 109receiver. The correlation output may be edited to remove interference.Additional information on the specifics of the digital editing techniquemay be found in the commonly assigned U.S. Pat. No. 6,765,531; “Systemand method for interference cancellation in a location calculation”

Geometric dilution of precision (GDOP) measures the sensitivity oflocation accuracy to the geometry of a TDOA and/or AoA system'sreceiving antennas relative to a transmitting mobile device. GDOP can beviewed as an error multiplier that can boost or degrade the performanceof a location system. For example, if the TDOA multi-lateration processhas measured every baseline in a location estimate with an accuracy ofX, and the geometry of the antennas resulted in a GDOP of Y, then theexpected error of the final location estimate is X*Y.

UTDOA location accuracy is highly dependent on obtaining a referencesignal obtained via partial demodulation and reconstruction by a limitednumber of LMUs and on selecting good cooperator sites. Limiting thenumber of LMUs is important because if all LMUs in the WLS were taskedto obtain a reference signal, those LMUs would be unavailable to performother locations during the same time period. This would severely impactthe availability of service (capacity and latency) of the WLS.

In order to improve reference signal detection without undue impact toavailability, a few LMU's (the demodulating LMUs) are tasked to attemptto demodulate the reference signal or portions of the reference signalsuch as the mid-amble and the best one is selected (or they arecombined) at a later time. The initial selection of which LMUs will betasked for demodulation or cooperation is static. The selection isperformed once during system configuration for each cell in the wirelesssystem.

While the selection of which LMUs will be tasked for demodulation orcooperation can take into account many aspects (such GDOP or thedistance between baseline pairs) in selecting the best sites, multi-pathpropagation and fading cannot be considered unless an extensive andexpensive drive test is performed after each system reconfiguration.Modeling of the radio propagation environment has thus far proveninsufficient to substitute for drive testing. In some cases the bestcells for detecting and demodulating the reference signal are not thecells that are close to the mobile of interest. Subsequent revision ofthe initial ‘static’ demodulation or cooperation lists as the WCNchanges or additional drive test and survey data become available ispossible. Revision of the lists using historical information in creatingthe static list is also possible (see commonly assigned U.S. patentapplication Ser. No. 11/948,244, Automated Configuration of a WirelessLocation System).

In an exemplary UMTS WCN, “Soft Handover” (SHO) is a state in which afew cells are simultaneously receiving and transmitting data to the samemobile device. A cell, in UMTS, corresponds to an antenna array with adefined radio coverage area. A UMTS cell may be an omni-directional cellor a sectored cell which uses a directional antenna array to define andserve the coverage area. One or more omni-directional or sectored cellsmay be sited at a single cell site.

With macrodiversity/soft handover the UE combines more than one radiodownlink from the local NodeB(s) to improve the reception quality.Although the maximum number of Radio Links that a UE can simultaneouslysupport is eight, the number used for SHO can vary dynamically in therange of 1 to 6 under the control of the radio access network.

In general, when an RRC connection is established, it first must beestablished on one cell. The UMTS network initiates Intra-Frequencymeasurements at the UE to determine if any other cells are suitable.Suitable cells have a CPICH (common pilot channel) strength (as measuredby Ec/Io) above the dynamic threshold value [Best_SS−AS_TH+AS_Th_Hyst]for deltaT seconds. If a cell CPICH meets this threshold and the ActiveSet is not yet full (or if the measured CPICH is better than an existingActive Set member's CPICH strength plus As_Rep_Hyst for deltaT seconds)then the cell is suitable.

When a suitable cell is found then Active Set Update procedure (3GPP TS25.331 “Radio Resource Control Protocol Specification”, section 8.3.4)is initiated. Using the Active Set Update message, the network adds ordeletes one (or more) radio link(s) to the UE. The only requirement isthat from the start until the end of this Active Set Update procedure,one Radio Link should remain common.

Since cells are added or removed from the Active Set based on RFmeasurement reports created at the mobile/UE, cells in the Active Setare likely to be very good candidates for demodulation and cooperationfor network-based wireless location methods.

Location Calculations in a TDMA/FDMA Wireless Communications Network

High precision network-based wireless location techniques that are fieldproven and in currently in wide usage include UTDOA (Uplink TimeDifference of Arrival), Angle of Arrival (AOA) and U-TDOA/AoA hybrids.

Uplink Time Difference of Arrival (UTDOA) determines a mobile phone'slocation by comparing the times at which a cell signal reaches multipleLocation Measurement Units (LMUs).

Angle of Arrival (AOA) determines a mobile phone's location by comparingthe angle developed by a multiple element antenna array in which theexact location of each antenna element is known precisely with the angledeveloped by another multiple element antenna. Each element is capableof separately receiving the uplink radio signal. By measuring signalstrength, time of arrival, and phase at each element of the array, it ispossible to calculate the line-of-sight path (bearing line) from themobile device to the AoA array. Use of two or more AoA equipped LMUsgenerates multiple bearing lines. A location estimate can be calculatedfrom where the bearing lines cross.

Network-based Hybrid Location solutions using combinations of Cell ID(CID), signal power measurements, Enhanced Cell ID (E-CID), Angle ofArrival (AOA), and Uplink Time Difference of Arrival (U-TDOA) can beused to increase location accuracy and yield in a geographic servicearea.

In a TDMA (Time Division Multiple Access), FDMA (Frequency DivisionMultiple Access), FDD (Frequency Division Duplex) system, such as GSM, aserving cell (generally the cell with the best measured radio signal asreceived by the mobile station) is identified to the WLS by the WCN orLink Monitoring System (LMS) along with radio channel information neededto tune the geographically distributed antennas connected to LMUreceivers. A serving cell is also known as a serving sector (in the caseof sectorized base stations) or as a serving antenna pair (a cellularsector nominally has at least a transmission antenna and receivingantenna). Use of receiver diversity adds additional receive antennas toa sector. In this application, the term sector will be used for theradio coverage area of a receiver antenna or diversity receiverantenna(s) regardless of the nature of the cell site (Omni-directionalor sectored). Thus “serving sector” will refer to the coverage area ofthe serving cell.

Prior to activation, an extensive design phase is typically conducted onthe network-based WLS, where optimal siting of LMUs (please see commonlyassigned United Station patent application Ser. No. 11/736,950; “SparsedU-TDOA Wireless Location Networks”, Filed Apr. 18, 2007; United Stationpatent application Ser. No. 11/736, 920; “Sparsed U-TDOA WirelessLocation Networks”; Filed Apr. 18, 2007; United Station patentapplication Ser. No. 11/736,902; “Sparsed U-TDOA Wireless LocationNetworks”; Filed Apr. 18, 2007; and United Station patent applicationSer. No. 11/736,868 “Sparsed U-TDOA Wireless Location Networks”; FiledApr. 18, 2007) and accuracy estimations over the geographic service areaare calculated. As part of the design phase, lists of predeterminedcooperative LMU receivers (coop or cooperator lists) and lists ofpredetermined secondary demodulation candidate LMUs (demod lists) aregenerated for each sector of the WCN.

The initial cooperator list and demodulator list generation processmodels the radio path loss between an LMU sector receiver antenna(s) anda theoretical point (a marker point or sample point). The modeled radiopath loss is used to determine the value of a quality metric for theradio path between each marker point and a receiver antenna. In thisexample, the extended COST231-Hata radio propagation model will be used.

One or more marker points are used to represent the coverage area ofeach cell. Marker point placement can be performed using a number oftechniques including uniform geographic distribution, selectiveplacement based on radio propagation mapping, and random placement via aMonte Carlo or other stochastic probability technique. Simple geometricplacement will be used herein to illustrate the initial markerplacement.

FIGS. 2 a and 2 b illustrate examples of representative marker pointscalculated from a simple geometric model. FIG. 2 a depicts anillustrative example of the representative marker points as calculatedfrom a simple geometric model overlaid on the coverage area of aomni-directional antenna (single cell) cell site 201. The (n=4) markerpoints 202 are distributed around the antenna site 206 at a constantradius 204 along (n=4) individual equally distributed radials 205.

FIG. 2 b depicts an illustrative example of the representative markerpoints as calculated from a simple geometric model overlaid on onesector of a six sectored cell site 203. The current sector 205 isoverlaid with a simple geometric model. The (n=4) marker points 204 arearranged along the midpoints of the nominal sector edges and at ⅓ thenominal sector depth.

Using geographic and wireless communications system specificationsand/or survey information, the location of each receiver antenna isdetermined. In the case of multiple receiver antennas in a cell, asingle representative point can be chosen for all of that cell's receiveantennas, or calculations can be performed as if each receive antennawere associated with a unique cell.

The geographical location and other cell site information for non-LMUcells are known in the SMLC and used in generation with cooperator anddemod sector lists for those non-LMU sectors. A non-LMU sector can onlybe a serving cell and not a cooperator or demod sector.

It is necessary to determine which LMU sectors to use for cooperationand/or demodulation in performing locations for any cell in the servingarea. The following is an example procedure for determining an initialcooperator and demod sector list for any cell using the marker pointsfor that cell determined by any of the above marker point placementmethods to represent its coverage:

For each cell in the service area, perform the following steps 1-4:

-   -   1. Select all other LMU sectors (target sectors) within a        defined range (this value may vary based on the network        propagation modeling and can be unique to each cell).    -   2. Define n representative coverage points (the marker or sample        points). In FIGS. 2 a and 2 b, the marker points are uniformly        distributed over the expected coverage area of the current cell.        In practice, four markers are typically used.    -   3. Compute the expected path loss from all the markers to each        of the selected target sectors using a selected radio        propagation model to represent the radio channel. This modeling        may be refined using drive test data to improve the model.    -   4. Average the results of the path loss modeling into a single        quality metric representing the predicted radio link quality        between the current cell and each selected target sector, and        save these values for later use. The type of averaging used can        be a simple arithmetic mean, a geometric average, or other        averaging calculation appropriate to the propagation model.        Averaging can be performed using any appropriate representation        of the path loss including a direct ratio or a logarithmic        representation (for example, in decibels) of that ratio.

FIG. 2 d depicts an example of a table that represents, for all pairsconsisting of one serving cell and one target sector, the averages savedin step 4.

To generate the cooperating sector list and demod sector list for a callserved by a cell C, perform the “spiral algorithm” detailed in thefollowing steps 5-10:

-   -   5. Subdivide the area surrounding C's receive antenna(s) into m        radial segments. A representative point such as the centroid of        C's coverage area or the location of the base station hosting C        may be used for the center of the radial pattern. This point may        be referred to as the central point. In practice, m=8 has been        shown to provide useful results in a variety of network        topologies. FIG. 2 c illustrates an example for a serving sector        where m=8, thus dividing the area surrounding the current sector        into octants. Each octant is then numbered clockwise, from        1-to-8.    -   6. For each octant, create a list, called the target sector        list, containing the quality metric values and sector        identification information for the target sectors of the current        cell using the information saved in step 4 above.    -   7. Starting from any octant, select the target sector with the        best quality metric in that octant and move it to the cooperator        listing from that octant's target sector list.    -   8. To increase spatial symmetry (and thus lower GDOP) during        initial passes around the current sector's representative point,        the pattern 1-4-7-2-5-8-3-6 (repeated as needed) or a similar        non-sequential pattern may be used in the selection of the next        octant. For each octant in turn, select the target sector with        the best quality metric and move it to the cooperator listing        from that octant's target sector list.    -   9. Repeat the selection of octants and the election of        yet-unelected target sectors to the cooperative receiver list        based on the best remaining quality metric until the target        number of cooperators is reached or no more target sectors        remain in any of the octants.    -   10. Note the path loss to the last target sector added. Examine        all LMU sectors previously added to the cooperator list and for        each such sector, add to the cooperator list every LMU sector        not already on the cooperator list connected to an antenna that        is located in close geographic proximity to its antenna and has        average path loss low enough to plausibly be capable of        performing a baseline measurement.    -   11. For the current cell, create the demod sector list using        that cell's D best target sectors (highest power or lowest path        loss) from the quality metric table. (The value of D varies        according to the network characteristics, but generally falls        between 2 and 10). Order this list by quality metric, such that        the first list entry has the highest quality metric, the second        list entry (if any) has the second-highest quality metric, and        so on. Examine all LMU sectors previously added to the demod        list and for each such sector, add to the demod list every LMU        sector not already on the demod list whose antenna is located in        close geographic proximity to its antenna, regardless of        estimated path loss.

Steps 5-10 can be performed any time before tasking LMUs for datacollection.

If Angle of Arrival (AoA) capability is installed for the current cell,then a separate AoA cooperator list will be generated by adding all AoAequipped LMU sectors within range (the AoA range varies on a per cellbasis) that contain any of the current marker points within thefootprint of the AoA antenna array's beam.

FIG. 2 c illustrates the segmentation of the area surrounding thecurrent cell 206. The segmentation takes place radiating from thecentral point 207. The central point can be the location of the cellantenna or the centroid of the sector (as calculated from theintersection of the sector bisector and the midpoint of the sectorreach). Each segment 208 209 210 211 212 213 214 215 is numbered here as1-8 so that the distributive effect of the spiral algorithm can beshown.

FIG. 2 d details the lists of calculated signal metric between eachserving cell and target LMU sector as determined by the average signalquality metric from each marker point within a serving cell.

FIG. 2 e illustrates the progress of the spiral algorithm as for eachserving cell, it steps through the radial segments (nominally octants)in the selection 1-4-7-2-5-8-3-6 order used to distribute cooperators toavoid high GDOP resulting from poorly distributed cooperators.

An exemplary initial cooperator list in a tabulated format is detailedin FIG. 2 f. For each cell included in the service area (the ServingCell), the cooperator list for that cell consists of a list ofcooperating LMU sectors, in the order they were selected by the spiralalgorithm.

An exemplary initial demod LMU sector list in a tabulated format isdetailed in FIG. 2 g. For each cell included in the service area (theServing Cell), the demodulator list for that cell consists of a list ofdemod LMU sectors in the order they were selected in step 11 above.

The described procedure selects initial cooperators in an order thatbalances priority between signal strength and GDOP. Signal strength isprioritized by selecting LMU sectors located within each segment in theorder of strongest predicted signal strength over the serving sector.GDOP is prioritized by sequencing through the octants while selectingcooperating receivers, thus ensuring that the cooperating receiverssurround the serving sector. Balance between these priorities isachieved by alternating in this way between selecting the best LMUsector in each octant and stepping to the next octant. Each cell has itsown order of favored LMU sectors, from most to least advantageous. Atlocation time, the Initial Coop List is used to determine which LMUreceivers and associated receiver antennas are considered for signalcollection and correlation.

For Location Calculations in a Wireless Communications Network withSoft-Handoff/Softhandover

In a WCN that supports soft-handoff/handover, such as a CDMA (CodeDivision Multiple Access) based system, which can be a FDD (FrequencyDivision Duplex) or a TDD (Time Division Duplex) system, the concept ofa serving cell or serving sector is more complicated. For anetwork-based Wireless Location System (WLS) to function in WirelessCommunications Networks (WCNs) that support soft-handoff/softhandover,cooperative (and demod) receiver selection for a TDOA, AOA, TDOA/AOA, orhybrid network-based or network-overlay Wireless Location System (WLS)must contend with one or more serving cells.

The mobile device may have multiple serving cells (also known as activeset members). Furthermore, each cell may have differing numbers oftransmission and reception antennas or, in TDD-based systems, use thesame antenna for transmission and reception.

When the active set contains a single member, the cooperative receiverselection procedure using the initial cooperator and demod sector listsas described above may be used.

When the active set contains more than one member, the methods disclosedherein may be used to select cooperators and demod LMU sectors to usefor signal collection. Two categories of methods are described herein,with several methods in each category presented in detail.

The first category of methods is called proxy methods, since thesemethods choose one member of the active set to use as a proxy servingcell. The second category of methods is called aggregate methods, sincethe composition of the active set is used in its entirety (i.e. in theaggregate) to select demod and coop sectors. In both categories, theselected cooperative and demodulating receivers are very likely toprovide good TDOA and AoA coverage for the mobile device.

Proxy Methods for Location Calculations in a Wireless CommunicationsNetwork with Soft-Handoff/Softhandover

When locating a call and the mobile device's Active Set contains morethan one member, the designer may elect to discard potentially usefulinformation provided by the composition of the active set and distillthe active set's data into a single serving cell for the purpose ofselecting cooperators. This single representative cell is deemed a proxyserving cell and the cooperative receiver selection procedures usingthis proxy serving cell may be referred to as proxy methods. Once aproxy has been selected, a cooperative receiver list based on the proxyis determined. Proxy methods retain information about the active set fordemod sector list generation because the best known demod sectorcandidates are those associated with the active set members themselves.Thus, all active set members having a connected LMU are included firstin the demod sector list before (thus at a higher priority than) otherdemod sectors. If the number of active set members having a connectedLMU is more than the limit D from Step 11 above, the number of activeset members takes precedence. (The value of D varies according to thenetwork characteristics, but generally falls between 2 and 10). Otherlower-priority demod sectors are selected based solely on the proxycell, using the procedure detailed in Step 11 above, up to but notbeyond the limit D.

Most of the work required to perform proxy cooperator selection can bepre-computed in the form of the static list for the cell used as aproxy. Real time tasks may comprise: (1) select a representative (proxy)serving sector and use its pre-computed coops, and (2) augment the proxycell's list of demod sectors by prepending all active set members thatare not already on the list. This class of techniques is especiallysuitable for mixed mode systems (such as GSM/UMTS) where multi-mode LMUsmay be locating both TDMA/FDMA and CDMA mobile devices.

FIG. 3 depicts a high-level procedure for proxy determination andlocation using the proxy serving cell.

During or prior to deployment, a radio propagation model with a networktopology which may be enhanced by the addition of geographic topography,building shadowing models, and drive test collected signal data iscreated for the service area 301. This radio propagation model is usedto determine the initial cooperator list and demod sector list for anycell in the WLS service area 302.

The deployed wireless location system is populated with the initiallists 303. These lists will be used in the case of one-way handoff(single active set member) and in the proxy sector technique.

At some time after deployment, the WCN or LMS detects a triggering event304; examples of triggering events include 9-1-1, 1-1-2 emergency calls.Additional triggers are detailed in commonly assigned U.S. patentapplication Ser. No. 11/150,414 “Advanced triggers for location-basedservice applications in a wireless location system”.

The WCN or LMS passes the triggering information and tasking informationto the WLS 305. The tasking information includes the active setmembership. If the WLS detects multiple active set members (the mobiledevice is in soft-handoff/softhandover), then the WLS may elect, basedon the designer or deployer's option, one of the three proxy methods306.

In some embodiments, the radio propagation modeling 301 may have beenperformed, but the generation of the initial coop and demod lists 302may not be performed until after tasking information has been received.The radio propagation modeling 301 and generation of the initial coopand demods 302 to pre-populate the WLS 302 may require lesscomputational load to be executed after tasking information has beenreceived and before signal collection can begin. Similarly, in someembodiments both the radio propagation modeling 301 and the generationof the initial coop and demod lists may not be performed until aftertasking information has been received. All such choices are a designer'soption.

Once in a proxy method, the WLS selects a proxy serving cell 306 andretrieves (or computes) the associated cooperator and demod sectorlists, adding any non-included LMU sectors associated with the activeset to the cooperator and demod sector lists. For tasking LMUs,membership in the active set may be a better indicator of usefulness asa cooperative receiver or demodulating receiver than using proximity asan indicator. The WLS collects radio signals via the specified LMUs 307.The WLS then uses the collected radio signals to calculate a locationestimate and velocity estimate using TDOA, TDOA with AoA, or via hybridtechniques 308.

Proxy Method 1—Select the Active Set Member Nearest to Centroid as theProxy

One way to select a proxy serving cell when the active set contains twoor more members is to find the cell closest to the centroid of theactive set members. The centroid may be found by averaging (separately)the x and y geographic coordinates of the individual sites. Alternately,the centroid can be calculated on a power or signal quality basis.

The centroid is a crude estimator of the UE's location, allowing theselection of a proxy cell whose coverage surrounds this point. Forexample, an omni-directional antenna's coverage area is centered at itsgeographic location so that when the closest cell to the centroid iswithin a predefined range and has an omni-directional antenna pattern,the closest cell can be expected to be a good choice for use as a proxyserving cell.

As shown in FIG. 4, it is possible that the cell 401 closest to thecentroid 413 is not a member of the active set. Rather than simplypicking the closest cell, the closest active set member is selectedbecause the active set members are chosen by the wireless network basedon actual received signal quality. Membership in the active set may be abetter indicator of usefulness as a cooperative receiver or demodulatorthan proximity. In FIG. 4, a mobile device 406 is shown in a 3-waysofthandoff/softhandover with sectors 410 411 412 via the radio links407 408 409. As shown, the involved sectors 410 411 412 are associatedwith base stations/cell sites 403 405 and 404 respectively althoughmultiple sectors associated with the same cell site can be involved. Theuninvolved sectors associated with cell sites 401, 402, 403, 404 and 405can all be potential cooperative or demod sectors.

Proxy Method 2—Select as Proxy Cell the Active Set Member with the MostActive Set Members on its Demodulator List.

In this method, the list of demod LMU sectors for each active set memberis examined to see if those demod sectors are themselves members of theactive set. If a given cell's demod list contains all the members of theactive set, then that sector is chosen as the proxy serving cell.

If no cell's demod sector list contains all active set members, but oneor more cells contain some members, then the cell with the greatestnumber of active set members in its demod list is chosen as the proxysector.

FIG. 5 shows a cellular network made of base stations 501 502 503 504505. A mobile device 506 is engaged in a soft-handoff with three sectors510 511 512 using radio links 507 508 509. In this method, the centroidis not calculated; rather the SMLC examines the demod list associatedwith each of the involved cells 510 511 512, and determines if any ofthese demod lists includes sectors 510 511 and 512. If the demod listfor any active set member cell 510 511 and 512 includes all other activeset members 510 511 and 512, then that member cell is selected as theproxy. Otherwise the active set member cell 510 511 and 512 having thedemod list that includes the most other active set members 510 511 and512 is selected as the proxy serving cell.

In case of a tie as to the most included active set members, one of theother proxy methods may be used to break the tie. Alternatively, anarbitrary (deterministic or random) selection of the proxy from theactive set may be made.

Proxy Method 3—Select Proxy Cell Based on Coverage Bounding Polygons

In this method a bounding polygon is defined for each sector that fullyencompasses all mobile locations at which that sector is expected toserve as an active set member.

During proxy selection, the intersection of the bounding polygons of allactive set members is used as a crude estimate of the mobile's position.The sector whose bounding polygon most closely matches the area of theintersection is chosen as the proxy sector. In practice, the sector withthe bounding polygon with the smallest area may be selected as proxy.

In FIG. 6, a geographical depiction of the bounding polygon selection ofproxy method is shown. The wireless communications network is depictedby the five sectored cells 601 602 603 604 605. The polygon shape usedin this example is a rectangle for simplicity of depiction. The boundingrectangles 610 611 612 are sized so as to encompass the entire area ofuseful radio coverage provided by the involved sectors 613 614 615. Theradio links 607 608 609 between the involved sectors 613 614 615 and thewireless device 606 are shown. Using the bounding polygon selectionmethod, sector 614 is selected as the proxy sector.

Aggregate Methods for Location Calculations in a Wireless CommunicationsNetwork with Soft-Handover

While proxy methods discard information in order to select a single cellas the proxy serving cell and allow use of the initial cooperator anddemod lists, aggregate methods for selecting cooperators and demodsectors use the information provided by active sets associated with morethan one member to generate new cooperator lists and demod lists basedon the composition of the whole active set. Aggregate cooperator anddemod sector selection requires computing cooperators in real-time basedon reported active set members, as the number of possible combinationsare extremely large thus making pre-computation difficult for all butthe smallest WLSs. This computation in real-time is processor intensivebut in most cases yields a better selection of cooperators and demodsectors as compared to proxy methods.

FIG. 7 illustrates the generalized operation of the aggregate methods. Aradio propagation model is created for the area 701 and the radiopropagation model is used to determine the initial cooperator list anddemod sector list for any cell in the WLS service area 702. The deployedwireless location system is populated with the initial lists 703. Atsome time, the WCN or the LMS signals that a location triggering eventhas occurred 704, and call-related information which includes the activeset details is passed to the WLS 705.

In some designs, the radio propagation modeling 701 may have beenperformed but the generation of the initial coop and demod lists 702 maynot be performed until after tasking information has been received. Theradio propagation modeling 701 and generation of the initial coop anddemods 702 to pre-populate the WLS 703 may require less computationalload to be executed after tasking information has been received andbefore signal collection can begin. Similarly, in some designs both theradio propagation modeling 701 and the generation of the initial coopand demod lists 702 may not be performed until after tasking informationhas been received. All such choices are a designer's option.

The WLS, using the newly acquired active set details computes newcooperator 706 and demod sector lists 707. The WLS then uses the newlycomputed lists to task the LMU network for signal collection 708. Usingthe reported time differences of arrival and/or angles of arrival, theWLS computes a final location, speed and heading 709 with errorestimates for each.

Aggregate Method 1—Construct New Cooperators in a Round-Robin FashionBased on Octants of Active Set Members.

This method is based on octant structures formed during coop generationfor a single serving cell, as described above. Every active set memberis segmented into an octant structure as though it were a single servingcell. Then, instead of spiraling around a single site, this methoditerates through the octant structures of all the active set memberswhile incrementing the octant number. The net effect is to take coopsfrom every active set member and a range of relative azimuths about eachone.

FIG. 8 a and FIG. 8 b illustrate the use of the active set membership indetermining the selection of potential cooperative and demod receiversand the selection of cooperative and demod receivers likely to provideTDOA and AoA coverage for the mobile device.

FIG. 8 a depicts a cellular network comprising base stations 801 802 803804 805. A mobile device 806 is engaged in a soft-handoff with threesectors 810 811 812 using radio links 807 808 809. In this aggregatemethod, the SMLC recalls the marker points and associated quality metricto every other potential cooperator for the involved cells 810 811 812.The area surrounding each of the involved cells 810 811 812 is thensegmented using the radial octant method detailed in FIG. 2 c. The SMLCthen recalculates a fresh cooperator and demod sector list by selectingthe best candidate cooperators based on the quality metrics associatedwith the sectors in the current radial segment (using eight radialsegments as in the example, each segment is one octant). Selection ofthe current segment is performed in a round-robin fashion by steppingthrough both the sectors and the octants. For example, the currentinvolved sector may be stepped through in sequential fashion (in FIG. 8b, an acceptable sector pattern would be 810-811-812-810-811-812 . . . )and the octants may be stepped through following the octant selectionpattern 1-4-7-2-5-8-3-6. Thus, for the first 24 cooperators selected thepattern of cells and octants may be as shown in the table in FIG. 8 c.The octant selection pattern 1-4-7-2-5-8-3-6 is exemplary and otherselection patterns may be used to provide geometric diversity or spatialsymmetry.

Selection of a fresh set of cooperators may be continued, repeating theselection of segments from the involved sectors and the election ofyet-unelected target sectors to the fresh cooperative receiver listbased on the best remaining quality metric until the target number ofcooperators is reached or no more target sectors remain. As in step 10above, once the stopping point is reached, the power of the last LMUsector added is determined. The LMU sectors previously added to thecooperator list is examined and, for each such LMU sector, the LMUsectors not already on the cooperator list that are connected to anantenna and is located in close geographic proximity to its antenna andhas average path loss low enough to plausibly be capable of performing abaseline measurement is added to the cooperator list.

A fresh demod sector list is then created. All active set members havingan LMU connected are included first in the demod sector list, before(thus at a higher priority than) other demod sectors. If the number ofactive set members having an LMU connected is more than the limit D fromStep 11 above, the number of active set members takes precedence. (Thevalue of D varies according to the network characteristics, butgenerally falls between 2 and 10). Next, all LMU sectors previouslyadded to the demod list are examined and, for each such sector, LMUsectors not already on the demod list whose antenna is located in closegeographic proximity to its antenna, regardless of estimated path loss,is added to the demod list. Other, lower-priority demod sectors areselected (at lower priority, and without duplication) in round-robinfashion from the demod sector lists of the individual active setmembers, as in the procedure detailed in Step 11 above but following thesame round-robin pattern used for coop selection and illustrated inFIGS. 8 a, 8 b and 8 c.

This aggregate method is relatively straightforward, but may not beoptimal because it indirectly uses the union of the coverage areas ofthe active set members rather than the intersection of those areas.Accordingly, this aggregate method may, in some instances, selectcooperative receivers that are not optimal candidates. Due to its simpleapproach, however, this method can still be useful in situations where alarge number of cooperative receivers must be accommodated. Like allaggregate methods, this method has the advantage that it incorporatesinformation about the coverage areas of all active set members in theselection of cooperating receivers and demod sectors.

Aggregate method 1 is equally applicable to the construction of coop anddemod sector lists for locating mobile units in wireless communicationsystems that incorporate distributed antenna systems (DAS). Adistributed antenna system uses multiple antennas typically located atseparate geographic sites to provide radio coverage for a single cell.The coverage area provided by a distributed antenna system is typicallysignificantly larger than the coverage area provided by one of itscomponent antennas.

To apply this method when there is a single serving cell and the receive(uplink) antenna system for that cell is a DAS, that cell's DAScomponent antennas may be substituted for the active set members ofAggregate Method 1 as though the active set consists of the DAScomponent antennas. Thus, marker points are used and the associatedpropagation metrics are calculated for each DAS component antenna andevery LMU sector that is connected to one of the DAS component antennasis elected as a demod sector. If the designer or deployer requires moredemod sectors than are part of the serving cell's DAS, others may beselected (at lower priority, and without duplication) in round-robinfashion from the demod sector lists of the DAS component elements usingthe procedure detailed in Step 11 above but following the sameround-robin pattern used for coop selection and illustrated in FIGS. 8a, 8 b and 8 c.

To apply this method when there is an active set consisting of more thanone cell and the receive (uplink) antenna system of at least one activeset member is a DAS, substitute for the active set members of AggregateMethod 1 the union of all LMU sectors associated with all the non-DASactive set members and all LMU sectors associated with all DAS componentantennas of all the DAS active set members, as though the active setconsists of all non-DAS active set members and the DAS components of allthe DAS active set members. Thus, marker points are used and theassociated propagation metrics are calculated for each non-DAS activeset member and for each DAS component antenna of any active set memberthat is a DAS and every LMU sector that is connected to a non-DAS activeset member or to one of the DAS component antennas of any active setmember is elected as a demod sector. If the designer or deployerrequires more demod sectors, others may be taken (at lower priority, andwithout duplication) in round-robin fashion from the demod sector listsof the DAS component elements, as in the procedure detailed in Step 11above but following the same round-robin pattern used for coop selectionand illustrated in FIGS. 8 a, 8 b and 8 c.

Aggregate Method 2—Construct New Cooperators List Based on Marker Pointsof Active Set Members

This method computes new coop and demod sector lists based on the unionof the marker points associated with the active set members. In anembodiment, the above described aggregate method may be extended bysegmenting the area surrounding the active set members using radialsegments centered at a representative point chosen based on geographicinformation associated with the active set members. In one embodiment,the centroid of the geographic locations of the active set members isused. The losses to each candidate sector from the marker points of theactive set members may be averaged.

FIG. 9 a illustrates a cellular network made of base stations 901 902903 904 905. A mobile device 906 is engaged in a soft-handoff with threecells 910 911 912 using radio links 907 908 909. In this aggregatemethod, the marker points are recalled for all involved cells and thequality metric between all target LMU sectors and each marker point iscalculated. The quality metric from each marker point for each involvedcell is then averaged so that each potential cooperator (target sector)has a single associated overall quality metric.

A representative central point for the area covered by the active set isthen chosen. In one embodiment, the centroid of the involved sectors isused. FIG. 9 b shows the centroid 913. The centroid is found byaveraging (separately) the x and y geographic coordinates of theindividual sector 910 911 912 receiver antennas. Alternate methods toselect a representative central point include calculating a weightedcentroid using weights based on a model of power or signal quality.

Once the central point 913 has been determined, the area surrounding thecentroid 913 is radially segmented (in this example, the centroid issegmented into octants). While following the octant selection pattern1-4-7-2-5-8-3-6, fresh sets of cooperators and demodulators are selectedby, for each octant in turn, selecting the target sector in the currentoctant with the best revised quality metric. Selection of a set ofcooperators is continued by repeating the selection of segments from theinvolved cells and the election of yet unselected target sectors to thecooperative receiver list based on best remaining quality metrics untilthe desired number of cooperators is reached or no more target sectorsremain for an octant. Once the stopping point is reached, the power ofthe last sector added is determined. The LMU sectors previously added tothe cooperator list are examined and for each such LMU sector, LMUsectors not already on the cooperator list that are connected to anantenna that is located in close geographic proximity to its antenna andhas average path loss low enough to plausibly be capable of performing abaseline measurement are added to the cooperator list.

A fresh demod sector list is then created. All active set members havinga connected LMU are included first in the demod sector list, before(thus at a higher priority than) other demod sectors. If the number ofactive set members having a connected LMU is more than the limit D fromStep 11 above, the number of active set members takes precedence. (Thevalue of D varies according to the network characteristics, butgenerally falls between 2 and 10). Next, examine all LMU sectorspreviously added to the demod list and for each such sector, add to thedemod list every LMU sector not already on the demod list whose antennais located in close geographic proximity to its antenna, regardless ofestimated path loss. Other lower-priority demod sectors are selectedusing the procedure detailed in Step 11 above, up to but not beyond thelimit D.

This method is more computationally intensive in that it requiresre-computing the path losses between all the marker points and thecandidate sectors, but having these new metrics based on the actualactive set members provides a better selection criteria for thecooperating sectors. By selecting each LMU sector based on an estimateof signal quality at marker points representing the coverage areas ofall active set members, this method has the potential to be more precisethan Aggregate Method 1. Like all aggregate methods, the methodincorporates information about the coverage areas of all active setmembers in the selection of cooperating and demodulating receivers.

Aggregate Method 3—Construct New Cooperators Based on the Coverage AreaCommon to all Active Set Members.

This method constructs new cooperative and demod receivers based onpredicting a common coverage area of all active set members. A new setof marker points is selected to represent the intersecting coveragearea. A representative central point for the intersecting coverage areais selected and the octant structure is placed at the central point. Theoctant structure is populated based on averaging the signal losses fromthese new marker points to each candidate sector. The above describedspiral algorithm is then used to select the cooperative receivers anddemod sectors.

As shown in FIG. 10 a, by using a bounding polygon to approximate thesector coverage area for each involved cell, a bounding polygon iscreated that fully encompasses the geographic area that the involvedcell serves.

First, the geographic area that falls into the bounding polygon of everyinvolved sector is determined. In FIG. 10 a, the area of geographicoverlap 1016 is determined by the bounding polygons (depicted in FIG. 10a as rectangles for ease of depiction). The wireless communicationsnetwork is depicted by the five sectored cells 1001 1002 1003 1004 1005.The bounding rectangles 1010 1011 1012 are sized so as to encompass theentire area of useful radio coverage provided by the involved sectors1013 1014 1015. The radio links 1007 1008 1009 between the involvedsectors 1013 1014 1015 and the wireless device 1006 are shown.

Once the area of overlap 1016 is determined, a set of h geographicallydistributed marker points is created. For each of the fresh markerpoints 1-to-h, the radio propagation model is used to determine aquality metric to every target sector within range. The range isadjustable and dependent on the network topology. The range may beexpressed as a distance or as a number of cell radii.

Once a fresh quality metric to every target sector within range for eachfresh marker point is generated, the quality metrics from each freshmarker point within the area of overlap 1016 are averaged to create asingle quality metric for each target sector. As shown in FIG. 10 b, thecentroid 1017 of the area of overlap 1016 is calculated and thegeographic area surrounding the centroid 1017 is radially segmented (inthis example, segmentation is again into octants). The radiallysegmented sections 1-8 are as shown in FIG. 10 b, centered on thecentroid 1017 of the area of overlap 1016 as calculated from thebounding polygons 1010 1011 1012 derived from the useful radio coverageareas of the involved sectors 1013 1014 1015. Following the octantselection pattern 1-4-7-2-5-8-3-6, a fresh set of cooperator and demodsectors are selected by, for each octant in turn, selecting the targetsector in the current octant with the best fresh quality metric.Selection of an updated set of cooperators is continued, repeating theselection of segments from the involved cells and the election of yetunselected target sectors to the fresh cooperative receiver list basedon best remaining quality metric until the desired number of cooperatorsis reached or no more target sectors remain. Once the stopping point isreached, the power of the last sector added is determined. The LMUsectors previously added to the cooperator list are examined and foreach such LMU sector, LMU sectors that are not already on the cooperatorlist connected to an antenna that is located in close geographicproximity to its antenna and has average path loss low enough toplausibly be capable of performing a baseline measurement are added tothe cooperator list.

A fresh demod sector list is then created. All active set members havinga connected LMU are included first in the demod sector list, before(thus at a higher priority than) other demod sectors. If the number ofactive set members having a connected LMU is more than the limit D fromStep 11 above, the number of active set members takes precedence. (Thevalue of D varies according to the network characteristics, butgenerally falls between 2 and 10). Next, examine all LMU sectorspreviously added to the demod list and for each such sector, add to thedemod list every LMU sector not already on the demod list whose antennais located in close geographic proximity to its antenna, regardless ofestimated path loss. Other lower-priority demod sectors are selectedusing the procedure detailed in Step 11 above, up to but not beyond thelimit D.

This method (and Proxy Method 3) assumes the determination of usefulbounds for sector coverage areas. If the bounds are too loose (meaningthat the polygons are larger than necessary), the coop and demod sectorselection suffer some loss of precision. If the bounds are too tight,the determination of an intersection may not be possible. A number oftechniques may be used to strike a balance between determining boundingpolygons that provide an intersection/overlap and yet not making thepolygons so large that the results are no longer useful). Suchtechniques include: 1) allowing more complex polygon shapes and/or 2)using multiple intersecting contours encompassing, e.g., 50%, 70%, or95% coverage areas, and analyzing the intersection areas to obtain acompact intersection polygon that encloses the likely mobile location.For cases in which a representative intersection cannot be found, othermethods may be used for fallback purposes.

Aggregate Method 3 incorporates the additional step of estimating thecoverage area of each cell and finding the intersection of those areas.Once that step is taken, this method has the advantage of requiringfewer path loss calculations as they are made from a smaller set ofmarker points than the union of all the marker points. The resultingmetrics may also provide better selection criteria for cooperativereceivers because they are computed from a better estimate of the likelylocation of the mobile device (the intersection of the coverage areas,rather than the union). This method is more complicated compared toAggregate Method 2 above but provides a closer to optimal representationof Active Set coverage. The trade-off is between computationalcomplexity and potential impacts to location latency vs. locationaccuracy.

CONCLUSION

The use of mobile generated radio quality information as expressed bythe generation of the Active Set for CDMA-based wireless communicationssystem represents a distinct improvement over other methods. Use of thedisclosed techniques allow for location accuracy improvement without anydrive test effort. The participating WLS receiver sites are selected inreal time based upon real RF channel conditions (taking into accountfading, temporary blocking, etc.) and system throughput is not impactedby over-selection of receiver sites in an attempt to improve accuracy.

The true scope the present invention is not limited to the presentlypreferred embodiments disclosed herein. For example, the foregoingdisclosure of a presently preferred embodiment of a Wireless LocationSystem uses explanatory terms, such as Serving Mobile Location Centers(SMLC), Location Measuring Unit (LMU), and the like, which should not beconstrued so as to limit the scope of protection of the followingclaims, or to otherwise imply that the inventive aspects of the WirelessLocation System are limited to the particular methods and apparatusdisclosed. Moreover, as will be understood by those skilled in the art,many of the inventive aspects disclosed herein may be applied inlocation systems that are not based on TDOA or AoA techniques. TheLMU's, etc. are, in essence, programmable data collection and processingdevices that could take a variety of forms without departing from theinventive concepts disclosed herein. Given the rapidly declining cost ofdigital signal processing and other processing functions, it is easilypossible, for example, to transfer the processing for a particularfunction from one of the functional elements (such as the SMLC)described herein to another functional element (such as the LMU) withoutchanging the inventive operation of the system. In many cases, the placeof implementation (i.e., the functional element) described herein ismerely a designer's preference and not a hard requirement. Accordingly,except as they may be expressly so limited, the scope of protection ofthe following claims is not intended to be limited to the specificembodiments described above.

Any of the above mentioned aspects can be implemented in methods,systems, computer readable media, or any type of manufacture. It shouldbe understood to those skilled in the art that the various techniquesdescribed herein may be implemented in connection with hardware orsoftware or, where appropriate, with a combination of both. For example,aspects of the invention may execute on a programmed computer. Thus, themethods and apparatus of the invention, or certain aspects or portionsthereof, may take the form of program code (i.e., instructions) embodiedin tangible media, such as floppy diskettes, CD-ROMs, hard drives, orany other machine-readable storage medium wherein, when the program codeis loaded into and executed by a machine, such as a computer, themachine becomes an apparatus for practicing the invention. In the caseof program code execution on programmable computers, the computingdevice generally includes a processor, a storage medium readable by theprocessor (including volatile and non-volatile memory and/or storageelements), at least one input device, and at least one output device.One or more programs that may implement or utilize the processesdescribed in connection with the invention, e.g., through the use of anAPI, reusable controls, or the like. Such programs are preferablyimplemented in a high level procedural or object oriented programminglanguage to communicate with a computer system. However, the program(s)can be implemented in assembly or machine language, if desired. In anycase, the language may be a compiled or interpreted language, andcombined with hardware implementations. In example embodiments acomputer readable storage media can include for example, random accessmemory (RAM), a storage device, e.g., electromechanical hard drive,solid state hard drive, etc., firmware, e.g., FLASH RAM or ROM, andremovable storage devices such as, for example, CD-ROMs, floppy disks,DVDs, FLASH drives, external storage devices, etc. It should beappreciated by those skilled in the art that other types of computerreadable storage media can be used such as magnetic cassettes, flashmemory cards, digital video disks, Bernoulli cartridges, and the like.The computer readable storage media may provide non-volatile storage ofprocessor executable instructions, data structures, program modules andother data for a computer.

Lastly, while the present disclosure has been described in connectionwith the preferred aspects, as illustrated in the various figures, it isunderstood that other similar aspects may be used or modifications andadditions may be made to the described aspects for performing the samefunction of the present disclosure without deviating therefrom. Forexample, in various aspects of the disclosure, various mechanisms weredisclosed for tracking a subject associated with a mobile device.However, other equivalent mechanisms to these described aspects are alsocontemplated by the teachings herein. Therefore, the present disclosureshould not be limited to any single aspect, but rather construed inbreadth and scope in accordance with the appended claims.

1. A method of identifying a set of cooperator and demodulator signalcollection receivers for use in locating a mobile device in a CodeDivision Multiple Access (CDMA)-based wireless communications network(WCN), comprising: obtaining data identifying an active set of basestations through which active communication is established between themobile device and the WCN; based on the active set, identifying at leastone cooperator receiver and at least one demodulator receiver; employingthe at least one cooperator receiver and the at least one demodulatorreceiver to collect signal data from the mobile device; and employingthe signal data to locate the mobile device.
 2. A method as recited inclaim 1, wherein demodulator receivers are identified based on apre-computed static list of cooperator and demodulator receivers forsectors corresponding to the base stations.
 3. A method as recited inclaim 2, further comprising generating the pre-computed static listbased on a radio propagation model for a service area.
 4. A method asrecited in claim 1, wherein the method is used in a mixed mode system(such as GSM/UMTS) in which multi-mode location measurement units (LMUs)are deployed for locating both TDMA/FDMA and CDMA mobile devices.
 5. Amethod as recited in claim 1, further comprising detecting a triggeringevent to initiate location of the mobile device, and communicatingtriggering information and tasking information to a wireless locationsystem (WLS), wherein the tasking information includes active setmembership information.
 6. A method as recited in claim 1, wherein saididentifying further comprises: for each base station, determining ageographic area associated with the base station and dividing thegeographic area into radial segments; and selecting, for each of theradial segments, cooperator receivers such that the selected cooperatorreceivers are substantially equally distributed among the base stationsand a range of relative azimuths about each base station.
 7. A method asrecited in claim 6, wherein said selecting further comprises: selectingcooperators based on a pre-computed static list of cooperator receiversfor a sector corresponding to the base stations in the active set.
 8. Amethod as recited in claim 6, wherein the radial segments are centeredon a geographic centroid of each base station.
 9. A method as recited inclaim 7, wherein the number of radial segments is eight.
 10. A method asrecited in claim 7, wherein said selecting comprises stepping throughthe radial segments using a spatially symmetric pattern and addingcooperator receivers to the cooperator receiver list based on anassociated quality metric for the radial segment.
 11. A method asrecited in claim 10, wherein said selecting further comprises cyclingthrough the base stations in the active set in conjunction with saidstepping through the radial segments.
 12. A method as recited in claim11, further comprising continuing the selection of cooperator receiversuntil a predetermined number of cooperator receivers is reached or allradial segments for the base stations in the active set have beenanalyzed.
 13. A method as recited in claim 12, further comprising: foreach added cooperator receiver, determining an average path loss for theadded cooperator receiver and adding cooperator receivers within apredetermined proximity and a predetermined average path loss.
 14. Amethod as recited in claim 6, wherein the WCN comprises a distributedantenna system (DAS) and at least one base station of the active setcomprises DAS antennas, further comprising using the DAS antennas as atleast part of the active set.
 15. A method as recited in claim 6,wherein the WCN comprises a distributed antenna system (DAS) and atleast one base station of the active set comprises DAS antennas, furthercomprising using as the active set the union of active set memberswithout DAS antennas and DAS antennas of the remaining active setmembers.
 16. A method as recited in claim 13, wherein the at least onedemodulator receiver is identified as a cooperator receiver within apredetermined proximity.
 17. A method as recited in claim 1, whereinsaid identifying further comprises: determining a representative pointbased on geographic information for the active set and determining ageographic area associated with the representative point; and dividingthe geographic area into radial segments and identifying at least onecooperator receiver and at least one demodulator receiver based on theradial segments.
 18. A method as recited in claim 17, wherein therepresentative point is a geographic centroid.
 19. A method as recitedin claim 18, wherein the geographic centroid is determined by averagingthe geographic coordinates of the base stations in the active set.
 20. Amethod as recited in claim 18, wherein the geographic centroid isdetermined as a function of signal power levels received from the basestations.
 21. A method as recited in claim 18, wherein the geographiccentroid is determined as a function of signal quality metricsassociated with the base stations.
 22. A method as recited in claim 17,wherein the number of radial segments is eight.
 23. A method as recitedin claim 17, wherein said selecting comprises stepping through theradial segments using a spatially symmetric pattern and addingcooperator receivers to the cooperator receiver list based on anassociated quality metric for the radial segment.
 24. A method asrecited in claim 23, wherein said selecting further comprises cyclingthrough the base stations in the active set in conjunction with saidstepping through the radial segments.
 25. A method as recited in claim24, further comprising continuing the selection of cooperator receiversuntil a predetermined number of cooperator receivers is reached or allradial segments for the base stations in the active set have beenanalyzed.
 26. A method as recited in claim 25, further comprising: foreach added cooperator receiver, determining an average path loss for theadded cooperator receiver and adding cooperator receivers within apredetermined proximity and a predetermined average path loss.
 27. Amethod as recited in claim 26, wherein the at least one demodulatorreceiver is identified as a cooperator receiver within a predeterminedproximity.
 28. A method as recited in claim 1, wherein said identifyingfurther comprises: determining a common coverage area for the active setand a geographic centroid for the common coverage area; dividing thecommon coverage area into radial segments centered around the geographiccentroid and identifying at least one cooperator receiver and at leastone demodulator receiver based on the radial segments.
 29. A method asrecited in claim 28, wherein said identifying further comprisesselecting cooperators based on a pre-computed static list of cooperatorreceivers for a sector corresponding to the base stations in the activeset.
 30. A method as recited in claim 29, further comprising generatingthe pre-computed static list based on a radio propagation model for aservice area.
 31. A method as recited in claim 28, wherein saiddetermining a common coverage area further comprises employing abounding polygon to approximate a geographic coverage area for each basestation and determining an area of overlap as the common coverage area.32. A method as recited in claim 30, further comprising determininggeographically distributed marker points within the common coverage areaand, for each of the marker points, using the radio propagation model todetermine a quality metric to sectors within a predetermined range ofthe common coverage area.
 33. A method as recited in claim 32, furthercomprising averaging the quality metrics from each marker point todetermine a quality metric for each sector.
 34. A method as recited inclaim 33, wherein the number of radial segments is eight.
 35. A methodas recited in claim 33, wherein said selecting comprises steppingthrough the radial segments using a spatially symmetric pattern andadding cooperator receivers to the cooperator receiver list based on anassociated quality metric for the radial segment.
 36. A method asrecited in claim 35, further comprising repeating the selection ofcooperator receivers until a predetermined number of cooperatorreceivers is reached or all radial segments have been analyzed.
 37. Amethod as recited in claim 36, further comprising: for each addedcooperator receiver, determining an average path loss for the addedcooperator receiver and adding cooperator receivers within apredetermined proximity and a predetermined average path loss.
 38. Amethod as recited in claim 37, wherein the at least one demodulatorreceiver is identified as a cooperator receiver within a predeterminedproximity.
 39. A system configured to identify a set of cooperator anddemodulator signal collection receivers for use in locating a mobiledevice in a Code Division Multiple Access (CDMA)-based wirelesscommunications network (WCN), the system comprising at least oneprocessor and at least one storage medium communicatively coupled tosaid at least one processor, the storage medium having stored thereincomputer-executable instructions for instructing the processor incausing the following steps: obtaining data identifying an active set ofbase stations through which active communication is established betweenthe mobile device and the WCN; based on the active set, identifying atleast one cooperator receiver and at least one demodulator receiver;employing the at least one cooperator receiver and the at least onedemodulator receiver to collect signal data from the mobile device; andemploying the signal data to locate the mobile device.
 40. The system ofclaim 39, wherein demodulator receivers are identified based on apre-computed static list of cooperator and demodulator receivers forsectors corresponding to the base stations.
 41. The system of claim 40,further comprising instructions for instructing the processor in causingthe step of generating the pre-computed static list based on a radiopropagation model for a service area.
 42. The system of claim 39,wherein the set of cooperator and demodulator signal collectionreceivers is identified in a mixed mode system (such as GSM/UMTS) inwhich multi-mode location measurement units (LMUs) are deployed forlocating both TDMA/FDMA and CDMA mobile devices.
 43. The system of claim39, further comprising instructions for instructing the processor incausing the steps of detecting a triggering event to initiate locationof the mobile device, and communicating triggering information andtasking information to a wireless location system (WLS), wherein thetasking information includes active set membership information.
 44. Thesystem of claim 39, wherein said identifying further comprisesinstructions for instructing the processor in causing the steps of: foreach base station, determining a geographic area associated with thebase station and dividing the geographic area into radial segments; andselecting, for each of the radial segments, cooperator receivers suchthat the selected cooperator receivers are substantially equallydistributed among the base stations and a range of relative azimuthsabout each base station.
 45. The system of claim 44, wherein saidselecting further comprises: selecting cooperators based on apre-computed static list of cooperator receivers for a sectorcorresponding to the base stations in the active set.
 46. The system ofclaim 44, wherein the radial segments are centered on a geographiccentroid of each base station.
 47. The system of claim 45, wherein thenumber of radial segments is eight.
 48. The system of claim 45, whereinsaid selecting comprises stepping through the radial segments using aspatially symmetric pattern and adding cooperator receivers to thecooperator receiver list based on an associated quality metric for theradial segment.
 49. The system of claim 48, wherein said selectingfurther comprises cycling through the base stations in the active set inconjunction with said stepping through the radial segments.
 50. Thesystem of claim 49, further comprising instructions for instructing theprocessor in causing the step of continuing the selection of cooperatorreceivers until a predetermined number of cooperator receivers isreached or all radial segments for the base stations in the active sethave been analyzed.
 51. The system of claim 50, further comprisinginstructions for instructing the processor in causing the step of: foreach added cooperator receiver, determining an average path loss for theadded cooperator receiver and adding cooperator receivers within apredetermined proximity and a predetermined average path loss.
 52. Thesystem of claim 44, wherein the WCN comprises a distributed antennasystem (DAS) and at least one base station of the active set comprisesDAS antennas, further comprising using the DAS antennas as at least partof the active set.
 53. The system of claim 44, wherein the WCN comprisesa distributed antenna system (DAS) and at least one base station of theactive set comprises DAS antennas, further comprising using as theactive set the union of active set members without DAS antennas and DASantennas of the remaining active set members.
 54. The system of claim51, wherein the at least one demodulator receiver is identified as acooperator receiver within a predetermined proximity.
 55. The system ofclaim 39, wherein said identifying further comprises: determining arepresentative point based on geographic information for the active setand determining a geographic area associated with the representativepoint; and dividing the geographic area into radial segments andidentifying at least one cooperator receiver and at least onedemodulator receiver based on the radial segments.
 56. The system ofclaim 55, wherein the representative point is a geographic centroid. 57.The system of claim 56, wherein the geographic centroid is determined byaveraging the geographic coordinates of the base stations in the activeset.
 58. The system of claim 56, wherein the geographic centroid isdetermined as a function of signal power levels received from the basestations.
 59. The system of claim 56, wherein the geographic centroid isdetermined as a function of signal quality metrics associated with thebase stations.
 60. The system of claim 55, wherein the number of radialsegments is eight.
 61. The system of claim 55, wherein said selectingcomprises stepping through the radial segments using a spatiallysymmetric pattern and adding cooperator receivers to the cooperatorreceiver list based on an associated quality metric for the radialsegment.
 62. The system of claim 61, wherein said selecting furthercomprises cycling through the base stations in the active set inconjunction with said stepping through the radial segments.
 63. Thesystem of claim 62, further comprising instructions for instructing theprocessor in causing the step of continuing the selection of cooperatorreceivers until a predetermined number of cooperator receivers isreached or all radial segments for the base stations in the active sethave been analyzed.
 64. The system of claim 63, further comprisinginstructions for instructing the processor in causing the step of: foreach added cooperator receiver, determining an average path loss for theadded cooperator receiver and adding cooperator receivers within apredetermined proximity and a predetermined average path loss.
 65. Thesystem of claim 64, wherein the at least one demodulator receiver isidentified as a cooperator receiver within a predetermined proximity.66. The system of claim 35, wherein said identifying further comprises:determining a common coverage area for the active set and a geographiccentroid for the common coverage area; dividing the common coverage areainto radial segments centered around the geographic centroid andidentifying at least one cooperator receiver and at least onedemodulator receiver based on the radial segments.
 67. The system ofclaim 66, wherein said identifying further comprises selectingcooperators based on a pre-computed static list of cooperator receiversfor a sector corresponding to the base stations in the active set. 68.The system of claim 67, further comprising instructions for instructingthe processor in causing the step of generating the pre-computed staticlist based on a radio propagation model for a service area.
 69. Thesystem of claim 66, wherein said determining a common coverage areafurther comprises employing a bounding polygon to approximate ageographic coverage area for each base station and determining an areaof overlap as the common coverage area.
 70. The system of claim 68,further comprising instructions for instructing the processor in causingthe steps of determining geographically distributed marker points withinthe common coverage area and, for each of the marker points, using theradio propagation model to determine a quality metric to sectors withina predetermined range of the common coverage area.
 71. The system ofclaim 70, further comprising instructions for instructing the processorin causing the step of averaging the quality metrics from each markerpoint to determine a quality metric for each sector.
 72. The system ofclaim 71, wherein the number of radial segments is eight.
 73. The systemof claim 71, wherein said selecting comprises stepping through theradial segments using a spatially symmetric pattern and addingcooperator receivers to the cooperator receiver list based on anassociated quality metric for the radial segment.
 74. The system ofclaim 73, further comprising instructions for instructing the processorin causing the step of repeating the selection of cooperator receiversuntil a predetermined number of cooperator receivers is reached or allradial segments have been analyzed.
 75. The system of claim 74, furthercomprising instructions for instructing the processor in causing thestep of: for each added cooperator receiver, determining an average pathloss for the added cooperator receiver and adding cooperator receiverswithin a predetermined proximity and a predetermined average path loss.76. The system of claim 75, wherein the at least one demodulatorreceiver is identified as a cooperator receiver within a predeterminedproximity.
 77. A computer readable storage medium storing thereoncomputer executable instructions for identifying a set of cooperator anddemodulator signal collection receivers for use in locating a mobiledevice in a Code Division Multiple Access (CDMA)-based wirelesscommunications network (WCN), said computer executable instructions for:obtaining data identifying an active set of base stations through whichactive communication is established between the mobile device and theWCN; based on the active set, identifying at least one cooperatorreceiver and at least one demodulator receiver; employing the at leastone cooperator receiver and the at least one demodulator receiver tocollect signal data from the mobile device; and employing the signaldata to locate the mobile device.
 78. The computer readable storagemedium of claim 77, wherein demodulator receivers are identified basedon a pre-computed static list of cooperator and demodulator receiversfor sectors corresponding to the base stations.
 79. The computerreadable storage medium of claim 78, further comprising instructions forgenerating the pre-computed static list based on a radio propagationmodel for a service area.
 80. The computer readable storage medium ofclaim 77, wherein the method is used in a mixed mode system (such asGSM/UMTS) in which multi-mode location measurement units (LMUs) aredeployed for locating both TDMA/FDMA and CDMA mobile devices.
 81. Thecomputer readable storage medium of claim 77, further comprisinginstructions for detecting a triggering event to initiate location ofthe mobile device, and communicating triggering information and taskinginformation to a wireless location system (WLS), wherein the taskinginformation includes active set membership information.
 82. The computerreadable storage medium of claim 77, wherein said identifying furthercomprises: for each base station, determining a geographic areaassociated with the base station and dividing the geographic area intoradial segments; and selecting, for each of the radial segments,cooperator receivers such that the selected cooperator receivers aresubstantially equally distributed among the base stations and a range ofrelative azimuths about each base station.
 83. The computer readablestorage medium of claim 82, wherein said selecting further comprises:selecting cooperators based on a pre-computed static list of cooperatorreceivers for a sector corresponding to the base stations in the activeset.
 84. The computer readable storage medium of claim 82, wherein theradial segments are centered on a geographic centroid of each basestation.
 85. The computer readable storage medium of claim 83, whereinthe number of radial segments is eight.
 86. The computer readablestorage medium of claim 83, wherein said selecting comprises steppingthrough the radial segments using a spatially symmetric pattern andadding cooperator receivers to the cooperator receiver list based on anassociated quality metric for the radial segment.
 87. The computerreadable storage medium of claim 86, wherein said selecting furthercomprises cycling through the base stations in the active set inconjunction with said stepping through the radial segments.
 88. Thecomputer readable storage medium of claim 87, further comprisinginstructions for continuing the selection of cooperator receivers untila predetermined number of cooperator receivers is reached or all radialsegments for the base stations in the active set have been analyzed. 89.The computer readable storage medium of claim 88, further comprisinginstructions for: for each added cooperator receiver, determining anaverage path loss for the added cooperator receiver and addingcooperator receivers within a predetermined proximity and apredetermined average path loss.
 90. The computer readable storagemedium of claim 82, wherein the WCN comprises a distributed antennasystem (DAS) and at least one base station of the active set comprisesDAS antennas, further comprising instructions for using the DAS antennasas at least part of the active set.
 91. The computer readable storagemedium of claim 82, wherein the WCN comprises a distributed antennasystem (DAS) and at least one base station of the active set comprisesDAS antennas, further comprising instructions for using as the activeset the union of active set members without DAS antennas and DASantennas of the remaining active set members.
 92. The computer readablestorage medium of claim 89, wherein the at least one demodulatorreceiver is identified as a cooperator receiver within a predeterminedproximity.
 93. The computer readable storage medium of claim 77, whereinsaid instructions for identifying further comprises: instructions fordetermining a representative point based on geographic information forthe active set and determining a geographic area associated with therepresentative point; and instructions for dividing the geographic areainto radial segments and identifying at least one cooperator receiverand at least one demodulator receiver based on the radial segments. 94.The computer readable storage medium of claim 93, wherein therepresentative point is a geographic centroid.
 95. The computer readablestorage medium of claim 94, wherein the geographic centroid isdetermined by averaging the geographic coordinates of the base stationsin the active set.
 96. The computer readable storage medium of claim 94,wherein the geographic centroid is determined as a function of signalpower levels received from the base stations.
 97. The computer readablestorage medium of claim 94, wherein the geographic centroid isdetermined as a function of signal quality metrics associated with thebase stations.
 98. The computer readable storage medium of claim 93,wherein the number of radial segments is eight.
 99. The computerreadable storage medium of claim 93, wherein said instructions forselecting comprises instructions for stepping through the radialsegments using a spatially symmetric pattern and adding cooperatorreceivers to the cooperator receiver list based on an associated qualitymetric for the radial segment.
 100. The computer readable storage mediumof claim 99, wherein said selecting further comprises cycling throughthe base stations in the active set in conjunction with said steppingthrough the radial segments.
 101. The computer readable storage mediumof claim 100, further comprising instructions for continuing theselection of cooperator receivers until a predetermined number ofcooperator receivers is reached or all radial segments for the basestations in the active set have been analyzed.
 102. The computerreadable storage medium of claim 101, further comprising instructionsfor: for each added cooperator receiver, determining an average pathloss for the added cooperator receiver and adding cooperator receiverswithin a predetermined proximity and a predetermined average path loss.103. The computer readable storage medium of claim 102, wherein the atleast one demodulator receiver is identified as a cooperator receiverwithin a predetermined proximity.
 104. The computer readable storagemedium of claim 77, wherein said instructions for identifying furthercomprises: instructions for determining a common coverage area for theactive set and a geographic centroid for the common coverage area;instructions for dividing the common coverage area into radial segmentscentered around the geographic centroid and identifying at least onecooperator receiver and at least one demodulator receiver based on theradial segments.
 105. The computer readable storage medium of claim 104,wherein said instructions for identifying further comprises instructionsfor selecting cooperators based on a pre-computed static list ofcooperator receivers for a sector corresponding to the base stations inthe active set.
 106. The computer readable storage medium of claim 105,further comprising instructions for generating the pre-computed staticlist based on a radio propagation model for a service area.
 107. Thecomputer readable storage medium of claim 104, wherein said instructionsfor determining a common coverage area further comprises instructionsfor employing a bounding polygon to approximate a geographic coveragearea for each base station and determining an area of overlap as thecommon coverage area.
 108. The computer readable storage medium of claim106, further comprising instructions for determining geographicallydistributed marker points within the common coverage area and, for eachof the marker points, using the radio propagation model to determine aquality metric to sectors within a predetermined range of the commoncoverage area.
 109. The computer readable storage medium of claim 107,further comprising instructions for averaging the quality metrics fromeach marker point to determine a quality metric for each sector. 110.The computer readable storage medium of claim 108, wherein the number ofradial segments is eight.
 111. The computer readable storage medium ofclaim 108, wherein said selecting comprises stepping through the radialsegments using a spatially symmetric pattern and adding cooperatorreceivers to the cooperator receiver list based on an associated qualitymetric for the radial segment.
 112. The computer readable storage mediumof claim 111, further comprising instructions for repeating theselection of cooperator receivers until a predetermined number ofcooperator receivers is reached or all radial segments have beenanalyzed.
 113. The computer readable storage medium of claim 112,further comprising instructions for: for each added cooperator receiver,determining an average path loss for the added cooperator receiver andadding cooperator receivers within a predetermined proximity and apredetermined average path loss.
 114. The computer readable storagemedium of claim 113, wherein the at least one demodulator receiver isidentified as a cooperator receiver within a predetermined proximity.